1. Field of the Invention
The present invention relates to a radiographic image reading method and a radiographic image reading apparatus used in an input system of a medical radiographic image.
2. Description of Related Art
In the medical field, a radiographic image such as X-ray is often used for diagnosing diseases. Hitherto a method of using so-called radiography that a phosphor layer (phosphor screen) is irradiated with radiation passing through a specimen to generate visible light with which a film using silver halide is irradiated in the same way as taking a normal photograph and development is performed after forming a latent image, has been generally used as a method for forming a radiographic image.
In recent years, a method that radiation passing through a specimen is absorbed by a photostimulable phosphor of a photostimulable phosphor plate, subsequently radiation energy stored in the photostimulable phosphor by absorption is emitted as photostimulated light by exciting the photostimulable phosphor by, for example, light or thermal energy and the photostimulated light is detected for imaging, has been proposed as a method for forming a radiographic image without using a film coated with silver halide. Here, the photostimulable phosphor plate is a form of a radiographic image conversion panel.
A radiographic image conversion panel represented by such a photostimulable phosphor plate is desired to have high sensitivity for radiation as much as possible and give an image with good image qualities (sharpness, graininess and the like). Hitherto study for supersensitization and improvement of image qualities of a radiographic image conversion panel has been progressed.
For example, JP Tokukaihei-5-150100A discloses a radiographic image conversion panel comprising a photostimulable phosphor layer formed on a support wherein a light transmittance (Tth) in the thickness direction and a light transmittance (Tfl) in the plane direction have a relationship of 0<Tfl/Tth≦0.7.
Owing to such a structure of a radiographic image conversion panel, photostimulable excitation light incoming to the radiographic image conversion panel at a certain angle penetrates deeply in the thickness direction and spread of the light in the plane direction is suppressed and thus the sharpness is improved.
JP Tokukaihei-6-230198A discloses a radiographic image conversion panel wherein a photostimulable phosphor layer comprising a column crystal and a surface of the photostimulable phosphor layer comprising tops of the column crystal is planarized.
Owing to such a structure of a radiographic image conversion panel, there is less unevenness in the surface of the photostimulable phosphor layer. Therefore scattering of photostimulable excitation light is suppressed and thus the sharpness of an image is improved.
JP Tokukai-2002-131493A discloses a radiographic image conversion panel comprising a phosphor sheet comprising a reflection layer, an excitation light absorbing layer A that absorbs excitation light and a photostimulable phosphor layer are laminated on a support in this order and a protect film protecting the sheet and comprising an excitation light absorbing layer B. This panel is configured so that each excitation light absorbing layer has less absorbance at a peak wavelength in photostimulated luminescence than in incident light (excitation light).
Owing to such a structure of a radiographic image conversion panel, deterioration of sharpness resulting from excitation laser light can be prevented by making absorbance of photostimulated luminescence at the luminescence peak wavelength less than absorbance of excitation light at the peak wavelength. Thereby, the sensitivity for radiation is improved and an image with excellent sharpness can be obtained.
However, the method for manufacturing a radiographic image conversion panel described in JP Tokukaihei-5-150100A requires the step in which a material that has a different optical refractive index or transmittance from the photostimulable phosphor is formed in the thickness direction like a wall in order to make the ratio of the light transmittance (Tth) in the thickness direction and the light transmittance (Tfl) in the plane direction 0<Tfl/Tth≦0.7.
For the purpose of planarization of the surface of a photostimulable phosphor layer, the method for manufacturing a radiographic image conversion panel described in JP Tokukaihei-6-230198A requires the step for a forming continuous membrane in which, in formation of a column crystal by a vapor phase deposition method, the column crystal is grown to a predetermined height and subsequently a vapor phase deposition condition is changed to a condition in which a continuous membrane is formed, and further the vapor phase deposition of the column crystal is continued.
For the purpose of providing difference between absorbance of excitation light in the photostimulable phosphor layer and absorbance of excitation light in the protect film, the method for manufacturing a radiographic image conversion panel described in JP Tokukai-2002-131493A requires the step in which an excitation light absorbing layer is provided for the protect film and meanwhile an excitation light absorbing layer that has no direct relation with photostimulated luminance is formed between the support and the photostimulable phosphor layer also.
As above, for the purpose of improving sensitivity for radiation and obtaining an image with excellent sharpness, any one of the above-described method for manufacturing a radiographic image conversion panel requires the step of processing the photostimulable phosphor layer or providing the photostimulable phosphor layer with the wall, the membrane, or the layer that has no direct relation with photostimulated luminance. Accordingly, there is a problem that a material and manufacturing technique for forming such a membrane or layer are required and a manufacturing step become complex.